I. Field of the Invention
This invention relates to a directional coupler configured in a corrugated waveguide for separating signals in two bands of frequencies while maintaining their polarization characteristics of any arbitrary nature unaltered in each band. This invention can be also considered to be a diplexing device which permits the polarization characteristics of any arbitrary nature to be translated without any change at each frequency band.
II. Background Information
As is well known, satellite communication systems operate through the use of two distinct and well defined frequency bands where the higher frequency band (uplink) carries signals from the earth stations to the satellite while signals are sent from the satellite towards the earth stations in the lower frequency band (downlink). Moreover, to achieve better utilization of the available frequency bands, the frequencies are, often, reused by means of orthogonal polarizations.
For such a frequency reuse mode of operation, a diplexing system employs a diplexer which fulfills the requirement for separation of signals in two frequency bands without loss of polarization characteristics by band selective transduction of orthogonally polarized modes. In order to preserve the polarization characteristics, the diplexing system ought to present, at the same time, a low return loss characteristics in both bands. Furthermore, often such a system is rated to handle in a transmit band a high level of microwave power, typically, going up to 10 KW in each orthogonal polarization of the reused frequency.
With the recent introduction of greater available bandwidth, which extends from 3.4 to 4.8 GHz (excluding the segment of 4.2 to 4.5 GHz) for the downlink and from 5.8 to 7.075 GHz for the uplink and with specifications on the electrical performance continuing to allow reuse of frequency, all the existing designs of the frequency reuse diplexers fall well short of operating satisfactorily in these extended bands. Among the presently known frequency reuse diplexers, the ones that use quasi-optic filters are potentially limited in terms of available bandwidth and degradation of orthogonality of polarization. The ones in waveguides without corrugations on the walls do not accommodate the above stated extended bands without either generation of unwanted higher order modes or creation of high return loss. Any of the above two phenomena contributes towards deterioration of the polarization isolation and hence precludes such type of structures. Finally, the ones which are so far known to have used corrugated structures, have an abrupt transition into a co-axially arranged waveguide configuration followed by a branching waveguide network to separate the receive band while maintaining its polarization properties. Apart from having inherently high insertion loss in the downlink, this type of structure in their presently known configuration are susceptible to overmoding and poor return loss characteristics for extended bands of operation.